This invention relates generally to the recovery of viscous petroleum from viscous petroleum containing formations. Major deposits containing such petroleum deposits are located in western Canada, United States and in Venezuela. The depths of such deposits range from surface outcroppings to several thousand feet. This invention is directed to the recovery of petroleum from deposits located where surface mining techniques are impractical, and the injection of steam into the formation via a multiplicity of closely spaced wells is uneconomic or impractical.
To recover petroleum from such deposits proposals have been made to drill bore holes in a generally horizontal direction within the viscous petroleum containing formation. Such horizontal bores may be drilled either by deviating a conventional well bore or by excavating a mined shaft to the desired depth, lowering drilling equipment therein, and drilling the horizontal shafts therefrom. Systems of the latter type are disclosed in U.S. Pat. No. 3,994,340 to Donald J. Anderson et al and in U.S. Pat. No. 4,020,901 to Peter Pisio et al. Such systems are expensive to install especially where the depth of the formation exceeds a few hundred feet. With highly deviated bore holes, and especially where the terminal portion of such bore holes extends for substantial distances in a generally horizontal direction, the problem of introducing equipment for circulating steam and for recovering produced petroleum has not been solved to my knowledge, since the forces of gravity do not provide advancing force in the horizontal bore. Additionally, portions of horizontal well bores often cave in or slough. Therefore, when introducing equipment for heating and producing petroleum therefrom it is very desirable to circulate liquid through such equipment and around the space between the equipment and the bore hole wall to remove any material sloughed off the bore hole walls.
Many systems have been devised for injecting steam directly into the formation via closely spaced wells using either steam drive or the "huff and puff" method. Such steam must be at a pressure higher than reservoir pressure so that it can be injected into the formation. As is well known, the more closely steam approaches the critical condition the less its latent heat. This is the amount of heat required to convert liquid water to vapor and also it is the amount of heat given up when the steam condenses. It is the heat of condensation which provides the principal heat energy serving to raise the temperature of a petroleum reservoir.
For example, steam at 300 pounds per square inch and at a temperature of 417.degree. F. has a latent heat value of 809 BTU per pound. At a pressure of 1500 pounds per square inch and a temperature of 596.degree. F. the latent heat is only 556 BTU per pound.
In order to efficiently and economically heat a reservoir at substantial depth, steam at a pressure well below the reservoir pressure at such depth should be used to effectively heat the petroleum sands. Therefore, for the treatment of such petroleum reservoirs by heating with steam at pressure below reservoir pressures, the steam must be circulated in a closed loop system. The rate of heat flow from a conduit filled with a heatedfluid to a surrounding oil reservoir is controlled by the coefficient of heat transfer from the conduit wall which is measured in BTU per hour per square foot per degree Farenheit temperature difference. A number of factors control an overall coefficient but one which concerns this invention is the kind of insulation surrounding the conduit and its thickness. A conduit surrounded with thick, efficient insulation will transfer heat rather slowly while thin, or no, insulation permits more rapid heat flow. Thus, in a reservoir penetrated by a bore hole containing a steam-filled conduit, a selected portion of the reservoir may be heated preferentially if the conduit in that portion has a high coefficient of heat transfer by comparison with other, more effectively, insulated parts of the conduit. The portion of the conduit extending from the surface to the petroleum containing reservoir should have a low coefficient of heat transfer.